A typical gas turbine engine includes a compressor, a combustion section, and a turbine. Air flows axially through the engine. The air is compressed in the compressor and emerges therefrom at an elevated pressure and temperature. The pressurized air enters the combustor through a plurality of openings therein. A supply of fuel is delivered into the combustor from an exterior fuel tank through a fuel supply system and is dispersed into the combustor chamber through a plurality of fuel nozzles. As the compressed air and fuel are mixed, an ignitor plug, disposed within the chamber, ignites the mixture thereby resulting in combustion thereof. The hot, gaseous products of combustion subsequently expand and drive the turbine, which in turn drives the compressor, and are ultimately exhausted from the engine to produce thrust.
As is well known, any combustion process results in the production of various emissions, including nitrogen-oxygen compounds. The level of production of nitrogen-oxygen compounds is a function of the engine power setting. A low power setting corresponds to idle power and approach, whereas a high power setting corresponds to take-off, climb, and cruise. During the high power setting, levels of nitrogen-oxygen compounds emitted by the engine are substantially increased over levels of nitrogen-oxygen compounds emitted by the engine during the low power setting.
The nitrogen-oxygen compounds are hazardous to the environment because they deplete the ozone layer. Since gas turbine engines produce the greatest amount of nitrogen-oxygen compounds at high altitudes, the depletion of the ozone layer is more immediate with gas turbine engines operating at high altitudes than from other combustion processes, such as the operation of automobiles at sea level. Thus, there is an ongoing effort to reduce the levels of nitrogen-oxygen emissions in aircraft gas turbine engines in order to preserve the ozone layer.
One approach to reduce the nitrogen-oxygen emission level is through the introduction of multi-stage combustion. U.S. Pat. No. 4,265,615 entitled "Fuel Injection System For Low Emission Burners", issued to Lohmann and Markowski, discloses a dual stage combustion section with a main burner and a pilot burner. At a low power setting, the engine operates only with the pilot burner. As the power setting increases, the main burner begins to operate, and continues to operate throughout the high power setting. As the engine returns to the low power setting, the main burner is turned off and only the pilot burner operates.
Although multi-stage combustion reduces the level of nitrogen-oxygen compound emissions, another problem comes to the forefront. In multi-stage combustion engines, a problem of coking within the intricate fuel supply system becomes more acute. Coking is a thickening of any residual jet fuel that is stagnant at a certain location within the fuel system passages. When stagnant fuel is heated, it solidifies and can actually plug the fuel supply system.
Although coking does occur in conventional engines with single stage combustors as the engine cools following operation thereof, multi-stage combustion engines are particularly susceptible to coking because fuel tends to stagnate and get heated within the system when only one stage is operating. Coking occurs in the main burner fuel supply system after the engine is switched from the high power setting, where the main burner is operating, to the low power setting, where the main burner is no longer operating. Although the main burner is shut off during low power and the fuel is no longer flowing therethrough, the engine still operates and maintains its high ambient temperatures. The residual fuel that remains in the main burner fuel supply system gets heated and can solidify, thereby plugging the fuel lines.
Coking is particularly problematic at a check valve disposed within the fuel delivery system because the check valve is attached to the engine and it tends to get very hot by means of thermal conduction from the engine. The check valve is critical to the fuel flow within the fuel supply system. If the check valve is plugged, the fuel cannot be delivered to the main burner, thereby impeding the overall performance of the engine.